I like the layout design/track plan... looks good.
I've built a lot of door layouts over the years. We have craft stores here that sell small sheets of brown foam, that I use to cut to size to make ties for the track. Which really reduces the rumble sound of the hollow core door.
Pertaining to question one, sounds like there might be a problem with the UC tracks? They're momentary operation, so there will be a voltage drop when you are activating them. Which is normal since they run off track power. Your turnouts all look to be manual, so there's no draw there. It may be that, when the 2 switch tracks are both straight, the power is then shut off to that inner pass, so then the UC tracks aren't getting power. Then when you operate the UC track, it's getting power from the ground, but not the hot rail (which ever way that manual switch track shuts off power... either ground or hot).
Personally I prefer to rewire my UC tracks to run off auxiliary power instead of track power. That way, no matter how slow the train is going, I always have the same power going to the UC track.
Same goes for my trackside accessories like crossing gates, banjo signals, etc. They're all wired from secondary voltage (not the track) and activated via a toggle (or momentary button) and not a insulated rail or weight trigger lock on. My feeling is, for a small layout I don't need the crossing gate going down on every single pass of the train. Having those items on buttons for activation instead of a insulated rail adds a little more participation on my part when running the trains.
Also, don't know if you're planning on drilling holes, but here's a tip for feeding wires through a hollow core door. When drilling a hole, try to hold the drill as straight as possible, so your hole goes as perfectly straight through as possible. Then use a thin sipping straw, and feed that through the two holes in each board surface of the hollow core door. Going through the top is easy... you might have to fiddle with the straw to find the bottom hole. Once you do, it's way easier to run the wire through the straw, than trying to find the holes with the wire alone.
Also, as much as I like those 1950's vintage manual 027 switches (because of that shut off power feature), I use the MPC era 027 switches with the large brown box. Because of the horizontal sliding lever that operates the switch, I can hook those up with nylon fishline, drill a couple holes, and operate the turnouts from the front of the layout... a poor man's electric turnout.
Then for yard areas, I've gone to the trouble to chop them down to just the footprint size of the track. On small door layouts, this is a huge help for gaining a little more space for other things. I wedge a piece of foam under the swivel rails, which I have found holds it in place, yet still allows operation - just via my finger to select either straight thru the turnout or through the curve.
Another pointer: I painted my whole door with an acrylic neutral gray. This became my "track ballast." Then I figured out a layout plan, and screwed the track in. Then at my leisure, I used green and brown acrylic paints to edge off the track road bed and become my ground area.
Here's a link of interest. It's a old thread, but might inspire you.